Ott the Production of Vapor. 79 



avulsion and detachment of the particles of vapor, the 

 more it is rarefied ; and the greater its depth, the more 

 copious will be the exhalation. But this takes place, 

 when the atmosphere has the greatest levity. Hence the 

 phenomenon that evaporation is the most abundant, when 

 the specific gravity of the atmosphere is the least. When 

 the depth of the vaporific stratum is small, the lower par- 

 ticles will be strongly drawn down and held by cor- 

 puscular attraction, so that the causes which produce a 

 separation will not be able to dislodge and generate pai'- 

 ticles of the larger size ; but when it has a greater thick- 

 ness, the attraction is so small as to be overcome at a 

 greater depth, and particles of vapor of a larger bulk will 

 be formed. The magnitude therefore of the particles, 

 will be in proportion to the depth of the stratum. To ac- 

 count for the different heights that the columns of vapor^ 

 observed by the gentleman at Pepperell, assumed, it may 

 be considered that when the first column arose, the air 

 was dense, and the vaporific stratum consequently of 

 small depth ; the particles of vapor would therefore be 

 small, and as their levity is in proportion to their small- 

 ness, they would mount up to a greater height. The 

 gravity of the atmosphere was considerably diminished 

 when the second column arose, and the consequent in- 

 crease in the depth of the vaporific stratum admitted the 

 formation of larger particles of vapor ; but as the specif- 

 ic gravity of vapor is proportional to the magnitude of 

 its particles, it must, on account of its gravity, take a 

 lower station agreeable to observation. 



